Glucose transport into brain is mediated by facilitative transporter proteins (GLUT). Three GLUT isoforms are found in brain: GLUT1 is the blood brain barrier transporter, GLUT3 is the predominant neuronal transporter, and GLUT5 is found in microglia. The relationships between the concentrations and kinetic properties of these transporters and acute and chronic changes in glucose utilization are being studied in two separate series of experiments: 1.Chronic dehydration and rehydration, 2.Chronic hypoglycemia and chronic hyperglycemia in diabetic rats.